User's Guide

SCSI Interface Description Page 13
Figure 2-5
Signals Used in Transfer from Initiator to Target
Synchronous Data Transfer
Both the Initiator and Target must send a SYNCHRONOUS DATA TRANSFER
REQUEST (SDTR) message in order to set up synchronous data transfer
parameters. The drives supports both SDTR WDTR extended (multibyte) messages.
The host may initiate an SDTR to establish a new synchronous data transfer
agreement or to end a previously arranged synchronous data transfer agreement
and return to asynchronous data transfer mode.
The following table shows the format for the SDTR message.
Byte Value Description
0
01h Extended message
1
03h Extended message length
2
01h Synchronous Data Transfer Request Code
3
m* Transfer Period (m x 4 nsec)
4
** REQ/ACK Offset
* Transfer Period can be any value between 0Ah and 64h. Exceptions to m x 4 nsec are
made in the following cases: 1) If the value is 0Ch, which is defined in SCSI-3 as a period
of 50 nsec, 2) If the value is 0Bh, which sets the period to 37.5 nsec, and 3) If the value is
0Ah, which sets the period to 25 nsec
.
** REQ/ACK Offset can be any value between 4 and 32.
The Transfer Period is the minimum time allowed between leading edges of
successive REQ pulses and of successive ACK pulses to meet the requirements of
the drive for successful reception of data. The host and DAT drive can transfer data
with larger, but never smaller, Transfer Period than specified in the SDTR message.